1. Field of the Invention
This invention relates to an idling speed control system for an internal combustion engine.
2. Description of the Prior Art
Generally it is preferred to maintain the idling speed of the internal combustion engine of a vehicle at a low speed of about 600 to 700 rpm from the viewpoint of fuel consumption and stability of the combustion in the engine. Thus, there have been known various idling speed control systems in which an actuator for controlling the opening angle of the throttle valve is provided and the actuator is controlled according to the difference between the actual idling speed and a desired idling speed determined according to the operating conditions of the engine such as the temperature of the cooling water and whether or not a cooler is in operation so that the actual idling speed is equalized to the desired idling speed. Further, when the rotational speed of the engine abruptly falls below a certain value for some reason, there arises the possibility of the engine's stalling. In Unexamined Japanese Patent Publication No. 55(1980)-75547 is disclosed an idling speed control system for an internal combustion engine in which the amount of the intake air is increased by a certain amount to increase the rotational speed of the engine when the rotational speed thereof abruptly falls, thereby preventing stalling of the engine.
However, the idling speed control system of the Unexamined Japanese Patent Publication involves a problem that if an excessive amount of air is abruptly fed to the engine, overshoot is apt to occur and the rotational speed of the engine abruptly changes to adversely affect the feeling of the driver, though it is preferred to substantially increase the amount of the intake air to sharply increase the rotational speed of the engine in order to effectively prevent stalling of the engine.